All down to the jetstream being in the right place but I recall a Laker DC10-10 in the late '70's with an actual flying time Bangor to Gatwick at 4 hrs 25 min. The computer flight plan showed 4 hr 20 min....I recall querying it at the time and requested our supplier to resend it - still came back with the same answer! On another date a DC10-30 achieved LAX-LGW in approx 8 hrs 45 mins.